Field Of The Invention
The present invention relates to skin moisturization and more specifically, to compositions capable of imparting a moisturizing effect to the skin and methods for their use.
Physiologically, the skin is an organ composed of several layers, of tissue, the most external of which is called the stratum corneum. Underlying the stratam corneum is a basal layer which in turn is followed by a layer known as the dermis. The entire skin organ functions as a primary moisture reservior for the body as well as a protective mantle.
The outermost layer, the stratam corneum, varies in thickness from about 15 microns to about 500 microns depending upon body location. The thinnest layer is usually associated with the face or the back of the hands while the thickest layers are associated with gripping or walking surfaces, i.e. the palms of the hands or the soles of the feet. This layer plays its most significant role in controlling the level of moisture in the skin and is composed of keratinized cells, a natural moisturizing factor, and lipids. All of these function together as a protective coating, as well as a moisture barrier to retain moisture within the skin.
The cells of the basal layer undergo systematic change from the normal cell structure to the keratinized layer of the corneum. During this change, biochemical protein breakdown products are formed, among which are pyrrolidone carboxylic acids, which are believed to function as natural moisturizing compounds. Below the basal layer lies the normal dermis of the skin that holds and serves as the transport means for water to the general area.
Water is extremely important to the proper physical condition and appearances of the skin. Dry and chapped skin is largely the result of an insufficient level of moisture in the stratum corneum. Dry skin is usually characterized by at least one of the following signs or symptoms:
(a) rough and flaky appearance, PA1 (b) reduced flexibility, or PA1 (c) surface fissures.
As previously stated, not all of the signs or symptoms need be present simultaneously and it is quite common for skin to be rough and flaky while still retaining normal flexibility, or the skin may be able to retain a smooth surface with concurrent loss of flexibility. Both conditions could be due to lack of moisture in the outermost layer. While dry skin is characterized by the above signs, their presence does not preclude other causes which could be the underlying cause of the condition. In general, however, soft, pliable and healthy skin cannot be maintained in the absence of the proper level of moisture in the stratum corneum.
The leverl of moisture in the skin is dependent upon a number of factors among which are the water binding potential of the stratum corneum, the rate at which water is supplied to the internal layers of the stratum corneum, and the rate at which water is lost from the skin via external evaporation. Under normal conditions the water content and vapor pressure of the epidermis are higher than those of the surrounding air which results in the evaporation of water from the skin surface. Replenishment of this lost water is normally carried out by the body; however, the ability to replenish this water diminishes with age resulting in the general loss of skin softness and pliability with maturation. Additionally, skin can become dry because of excessive loss of water due to exposure to low humidities, continuous use of soaps or detergents, and contact with solvents or anhydrous materials. With these factors in mind, investigators have for a long time been actively searching for ways to maintain proper levels of moisture in the skin.
Historically, formulations can be traced back to the second century for a cream named "Ceratum Galeni" which comprises a mixture of molten bees wax and perfumed olive oil in which was incorporated the maximum quantity of water. More recent classical examples of emollient creams include the USP (United States Pharmacopiea) "Unguentum Aquae Rosae." This mixture's composition was reported as early as 1820 to contain about 45 percent rosewater, 40 percent expressed almond oil, 12 percent spermaceti and 3 percent bees wax. A general treatment of this historical development of skin care treatment products can be found in Cosmetics Science and Technology; Sagarin et al.; Interscience Publishers, Inc.; New York, New York; 1957, pages 81-182 which are incorporated herein by reference.
Since there appears no doubt that the water content of the stratum corneum is a very important factor in maintenance of normal soft flexible skin, most efforts have been directed to means for replenishment of this water from an external source. The treatment of dry skin involves the use of basic ingredients that have come to be known as emollients. Although the term has gained wide application, an emollient is generally defined as an agent which when applied to the skin will effect a softening of dry inflexible corneum by inducing rehydration.
While the evidence points to the fact that water alone might suffice as a treatment agent there has been great difficulty in achieving any significant result via this agent alone. The difficulty arises in the application itself since only a very thin film of water can be made to adhere to the skin. Due to the competing physical forces, evaporation will take place before any emollient effect can take place. Prolonged immersion in large quantities of water to overcome the problem of evaporation presents the danger of over hydration resulting in swelling of the corneum, possible cellular damage and even exacerbation of the original condition.
These problems have been overcome by incorporating water in various cosmetically acceptable bases that will allow availability of water to the stratum corneum and regulate water take-up. A presentation of examples of the kinds of compositions that have been developed is set forth in Cosmetics Science and Technology, Supra.
While many of the raw materials employed in these formulations possess emollient potential, the ingredients are usually classified on the basis of the particular feature of that ingredient in the overall formulation. In general the catagories comprise:
______________________________________ 1. Emollients 5. Emulsifiers 2. Barrier Agents 6. Preservatives 3. Healing Agents 7. Perfume Oils 4. Humectants 8. Coloring Agents ______________________________________
Humectants were originally employed to control moisture exchange between the product and the air, both in storage and on the skin. The earliest of hand-treatment products, "glycerine and rosewater", was simply a mixture of humectant (the glycerine) and water (the rosewater). While many agents posses sufficient humectant properties to ensure against water loss in the product, only three have found wide acceptance in commercial skin preparations. These include glycerol, propylene glycol, and sorbitol. The compounds are similar in that they are all polyhydric alcohols; however, their behavior in skin care products is quite dissimilar.
While the ability to prevent drying of the preparation itself may be important for aesthetic reasons such as consumer acceptance, it is more desirable to incorporate humectants that also possess the ability to impart an emollient effect. It should be noted that the presence or ability of the humectant to stabilize the formula, i.e., prevent water loss from the composition, is not indicative or anticipatory of an emollient effect on the skin by the same agent.
Typical humectants used in the past did decrease the rate of water loss from the vehicle itself and did prevent crust formation; however, they did not do one of two things: decrease the rate of water loss from, or increase the water content of, the stratum corneum..sup.1 FNT 1. Shelmire Jr., J. B.; "Archives of Dermatology," 82:24-31, 1960.